The present invention relates to pulse columns used in solvent extraction processes, and more specifically to a control device for use in regulating and monitoring the operation of a pneumatic pulser designed to provide pulse activity in a nuclear reactor fuel by-product extraction system.
Fuel elements, or assemblies, discharged from nuclear reactors contain significant quantities of uranium and plutonium isotopes which are still fissionable and present an energy source which is desirable to recover in many cases. The most common method utilized to reclaim this fissionable material is by dissolving the spent fuel and then passing the aqueous dissolution product through a solvent (liquid-liquid) extraction process. The heavy metals (uranium and plutonium) are thus separated from other fission products and impurities during the solvent extraction process.
In a typical solvent extraction process used in the separation of radioactive heavy metals from an aqueous solution, the radioactive solutes ordinarily enter the system in an aqueous phase. At least some of the solutes are extracted into an organic phase, sometimes called the solvent. The organic phase or solvent may consist of a single substance, but frequently it contains one or more extractants and may include a diluent and sometimes a diluent modifier. The extracted solutes are subsequently removed from the organic phase by adjustment of chemical conditions such that stripping, also known as back extraction, occurs into an aqueous phase separate from the original feed stream.
Typical solvent extraction apparatus may be described as a series of interconnecting chambers in a linear arrangement or cascade. The aqueous phase is fed into the cascade at one end and the organic phase is fed into the cascade at the opposite end. Thus the aqueous phase and the organic phase move through the cascade in a continuous and counter-current flow pattern, with the aqueous and organic components interacting with each other in each chamber. In each chamber of the cascade, a portion of the desirable fission by-products is extracted into the solvent and thus removed from the aqueous phase. The cascade is designed so that the aqueous phase inlet and organic phase outlet are at the same end, and the aqueous phase outlet and the organic phase inlet are located together at the opposite end. At the aqueous phase outlet end, substantially all of the desirable products have been removed from the aqueous phase. Further, at the organic phase outlet end, the organic phase is withdrawn from the cascade in a substantially loaded condition, with the desirable heavy metal by-products contained therein. Subsequent chemical processing operations are used to further separate the fissionable products from the solvent solution.
Among the mechanisms generally used to practice solvent extraction is the pulse column. A pulse column is a liquid-liquid contactor having a generally vertical cylindrical body or tower in which the rate of mass transfer is enhanced by hydraulic pulsation of the liquids in the column through a series of perforated plates. In conventional pulse columns, a rapid reciprocating motion of relatively short amplitude is applied to the liquid contents of the column. An air pulser is normally employed to power this reciprocating motion and the consequential interaction of the aqueous and organic phases. Air pulse agitation has been found to give improved rates of extraction and to reduce tower heights compared to the dimensions of the former packed column type of apparatus.
Two major extraction parameters are affected by pulse energy: total volumetric throughput and mass transfer efficiency. The pulse action forces organic solution upward through the plates and simultaneously pushes the aqueous phase downward Pulse energy supplied to the column is a function of frequency and amplitude, and the maintenance of designated frequency and amplitude values is critical to efficient pulser operation. As pulse energy is increased, total throughput increases to a maximum and then decreases. Unstable pulser operation can be identified by localized solvent/aqueous phase inversions along the length of the column. If such inversions become large enough, complete flooding of the column may occur. The most efficient mass transfer is obtained when interfacial solvent/aqueous phase area, formation of new interfacial area, and turbulence are maximized.
Conventional pulse column control units measure a peak amplitude of the pulse and frequency and combine these two values in linear fashion to obtain a resultant value in column inches. Such measuring devices only measure a "peak" or "valley" of a frequency curve or a pulse curve, and do not have any mechanism for measuring a duration of a pulse, i.e. narrow pulses can be given the same amplitude value as long pulses. Thus, by following the peaks only of the pulse amplitudes, conventional pulse monitoring and control devices often obtain inaccurate results. If the pulsing operation is not monitored accurately, the potentially damaging and inefficient consequences described previously may easily result.
Thus, it is evident that accurate knowledge of the frequency/amplitude product is of great concern in achieving acceptable pulse column operation. As such, there is a need for a device capable of accurately monitoring pulse amplitude and frequency, as well as pulse duration.